Magnetic liquids, which are commonly referred to as "ferrofluids", typically comprise a colloidal dispersion of finely-divided magnetic particles, such as iron, .gamma.-Fe.sub.2 O.sub.3, magnetite and combinations thereof, of subdomain size (for example, 10 to 300 Angstroms) in a liquid carrier. The dispersion of the particles is maintained in the liquid carrier by a surfactant which coats the particles. Due to the thermal motion (Brownian movement) of the coated particles in the carrier, the particles are remarkably unaffected by the presence of an applied magnetic field or other force fields, such as centrifugal or gravitational fields, and remain uniformly dispersed throughout the liquid carrier even in the presence of such fields.
A typical ferrofluid may consist of the following volume fractions: 4% particles, 8% surfactant and 88% liquid carrier. Ferrofluids are often named for the liquid carrier in which the particles are suspended because it is the dominant component. For example, a water-based ferrofluid is a stable suspension of magnetic particles in water, whereas an oil-based ferrofluid is a stable suspension of magnetic particles in an oil (such as a hydrocarbon, an ester, a fluorocarbon, a silicone oil or polyphenyl ether, etc.) In addition, as mentioned above, the surfactants for water- and oil-based ferrofluids are different.
Ferrofluid compositions are widely known, and typical ferrofluid compositions are described, for example, in U.S. Pat. No. 3,531,413. The magnetic particles which form a ferrofluid typically are comprised of an iron oxide. Oxide ferrofluids are highly stable in contact with the atmosphere, although ferrofluids containing metallic particles of Fe, Ni, Co and alloys thereof are also known in the art. Such ferrofluids compositions are utilized in a wide variety of applications, including audio voice-coil dampening, voice-coil cooling, inertia dampening, stepper motors, noise control and vacuum device seals. Other applications pertain to material separation processes and the cooling of electrical equipment.
A number of books and references discuss the science of magnetic fluids, including their preparation. These references include: Magnetic Fluid Applications Handbook, editor in-chief: B. Berkovsky, Begell House Inc., New York (1996); Ferrohydrodynamics, R. E. Rosensweig, Cambridge University Press, New York (1985); Ferromagnetic Materials-A Handbook on the Properties of Magnetically Ordered Substances, editor E. P. Wohlfarth, Chapter 8, North-Holland Publishing Company, New York and "Proceedings of the 7.sup.th International Conference on Magnetic Fluids", Journal of Magnetism and Magnetic Materials, Vol. 149, Nos. 1-2 (1995).
Ferrofluids were originally manufactured by grinding magnetic materials in the presence of a solvent, such as a normal alkane, and a surfactant, such as oleic acid.
Typical manufacturing processes for these ferrofluids are described in U.S. Pat. No. 3,215,572 and in an article entitled "Ferrohydrodynamic Fluids for Direct Conversion of Heat Energy", R. E. Rosensweig, J. W. Nestor and R. S. Timmins, Materials Associated with direct Energy Conversion, Proc. Symp. AlChE-IChemE, Ser. 5, pp. 104-118, discussion, pp. 133-137 (1965). In these ferrofluids, the magnetic particles are prevented from agglomerating by the mechanism of steric repulsion, which mechanism is well-known to one skilled in colloid science.
The grinding operation is conventionally carried out in a ball mill. However, a conventional ball milling operation takes anywhere from two to six weeks to complete.
The colloid formed by this process generally includes uncoated particles and large aggregates and thus requires a subsequent refinement in which undesirable particles and aggregates are removed. Moreover, the finished product often has a high viscosity due to the presence of small particles produced during the grinding process. Consequently, the yield is poor, preparation times are long and the associated costs are high.
Ferrofluids can also be manufactured by chemical precipitation as disclosed in U.S. Pat. No. 3,764,540. The ferrofluids produced in this latter manner are sterically stabilized with adsorbed surfactant. Another manufacturing process is disclosed in U.S. Pat. No. 4,329,241 which illustrates ferrofluid synthesis in an aqueous medium of particles stabilized by charge repulsion.
However, chemically-precipitated ferrofluid manufacturing techniques create chemical waste, comprising un-reacted metal salt solutions and uncoated particles in aqueous and nonaqueous media which must be disposed of in proper compliance with environmental regulations. The waste removal adds to the cost of manufacturing the ferrofluids.
U.S. Pat. No. 3,764,540 discloses ferrofluid compositions comprising stable suspensions of magnetite and elemental iron and a method for their manufacture. The disclosed manufacturing method involves comminuting a non-magnetic or an anti-magnetic precursor material to colloidal size and dispersing the comminuted precursor in a carrier fluid. Thereafter, the precursor material is converted to a ferromagnetic form. The disclosed precursor material is a sub-oxide of iron (called a Wustite composition) having the formula Fe.sub.1-x O wherein x has a value of 0.01 to 0.20. Conversion of this precursor material to a ferromagnetic material is accomplished by heating the colloidal mixture to temperatures in the range of about 200-570.degree. C.
A co-pending patent application, filed on Feb. 10, 1998, by Kuldip Raj and Lutful Aziz and assigned Ser. No. 09/021,228, now U.S. Pat. No. 5,958,282, describes the production of low-cost magnetic fluids utilizing water as a carrier liquid. In accordance with the disclosure of that application, a mixture of non-magnetic iron oxide particles (.alpha.-Fe.sub.2 O.sub.3 ), deionized water and surfactant is ground in an attritor mill with the surprising result that a stable, magnetic colloidal dispersion is obtained after a short period of grinding.
However, water-based ferrofluids are not suitable for many applications. Accordingly, there is a need for a process which produces an inexpensive oil-based ferrofluid which can quickly be manufactured in large volumes. It is further desirable that the ferrofluid be produced with a process that generates little or no waste and is not labor intensive.